The invention relates to the technical field of using vibratory waves to monitor and inspect the soundness or the integrity of mechanical structures in the general sense.
The term “mechanical structure” should be understood as meaning solid structures that are suitable for being inspected by vibratory waves, e.g. for use in the nuclear field, or more generally in the energy field, in the rail transport field (engineering works such as bridges, . . . ), or in the maritime field (ship structures), or in aviation for inspecting aerostructures.
The invention finds a particularly advantageous application in detecting and locating local defects (such as localized breaks or cracks) or local changes of state in mechanical structures (e.g. engineering works) that appear during their period of utilization.
Another particularly advantageous application for the invention lies in detecting and locating defects present in mechanical structures at the end of their fabrication process.
In the prior art, numerous devices are known for inspecting parts by ultrasound operating in transmission or in reflection and using an array of transducers as a source and as a receiver.
A major difficulty arises from the fact that the echo reflected on the inlet interface to the structure for inspection is much stronger than any echoes that might come from defects, such that the interface echo masks the echoes that need to be identified. This problem is made worse when the inspected part is complex in shape and/or non-uniform in structure.
In an attempt to remedy that drawback, patent FR 2 683 323 proposes a technique serving to limit the impact of the main echo associated with the interface between the transducer and the part, by using an ultrasound amplification technique with time reversal as described in document EP-A-0 383 650. In that technique, provision is made in a first stage to illuminate the zone under study from one or more transducers and to record the echoes coming from the part. In a second stage, the received signals are re-emitted after reversing their time distributions and possibly their waveforms. Thus, the last-received signals are the first to be re-sent. On the same lines, patent FR 2 696 573 proposes identifying defects by a time-reversal technique relying on successively re-emitting echoes measured with a set of transducers in order to reveal the greatest reflector. The publication by S. Granger et al.: “Monitoring of cracking and healing in an ultra-high performance cementitious material using the time-reversal technique” XP 0 260 378 51 describes a method relying on time reversal of ultrasound waves consisting in analyzing the variation caused by the appearance of a defect in the signal focused on the initial emission point. That analysis is performed using a transducer situated at the focus which is assumed to be close to the zone in which the defect appears.
Whatever the quality of the signal processing techniques used, known techniques for detecting or locating defects that rely on amplified re-emission of echoes measured on a structure suffer from problems associated with the complexity of most of the structures under study. The echoes associated with the looked-for defects are frequently found to be drowned among the multiple echoes created by singularities in the very structure being studied. Furthermore, some of those techniques require transducers to be installed close to the zones where defects appear, which can sometimes be a problem in terms of accessing such zones, and a high ratio is required between the number of transducers used and the number of zones to be monitored.